At the present time there are many instruments made for use in endoscopic medical procedures. Typically, endoscopic instruments are long and flexible cylindrically tubular devices with manually operated handles at their proximal ends and tissue-manipulative cutting, grasping, injecting, or cauterizing components at their distal ends. These distal devices, also called effector-end assemblies, after being moved to the intended treatment site by means of the endoscopic instrument tube, are operated by a control member, such as a wire. The control member typically may be pushed as well as pulled, to allow motion of the control element, and therefore operation of the distal device, in both directions. For example, the control member may allow a physician to close and open a pair a forceps by moving one of the forceps jaws relative to the other, which may be fixed. Alternatively, a biopsy or other needle may penetrate tissue by being inserted into such tissue at the distal end of the endoscopic instrument tube by pushing the control member distally by means of the control member's proximal handle. Further, a snare may be included as the effector-end assembly as disclosed in commonly assigned U.S. patent application Ser. No. 09/716,776, entitled Polypectomy Snare Having Ability to Actuate Through Tortuous Path,” filed on the same day herewith, the disclosure of which is hereby incorporated by reference.
The endoscopic instruments are introduced into a flexible endoscope which is inserted into the patient through a natural or surgically-created opening. The endoscope includes an elongate portion defining several lumens therethrough and a proximal handle for directing the elongate portion. At least one lumen is provided with an optical imaging system (e.g., a scope), and several lumina or “working channels” are typically provided for extending endoscopic instruments therethrough. The working channel of the endoscope typically consists of a PTFE-lined cylindrical tube passing from the proximal (handle) end of the endoscope to its distal (working) end. Working channels are typically 2 to 4 millimeters in inside diameter.
During the medical procedure, the doctor passes one or more endoscopic instruments through the working channel or channels in order to manipulate the tissue being visualized by the optical system of the endoscope. In the course of positioning the distal effector end assembly, usually the doctor must repeatedly manipulate the distal end of the instrument by manually pushing and pulling on the proximal portion of the tubular shaft of the endoscopic instrument near where the shaft enters the handle of the endoscope. After the end effector assembly has been placed at the treatment site, the end effector assembly must similarly be manipulated or effected using the control element. For example, the physician may wish to open or close a forceps, or insert a needle into tissue at the distal end of the endoscopic instrument and then withdraw the needle without moving the entire tubular shaft of the needle instrument.
The view through an endoscope is highly magnified when seen on the video monitors typically used for these procedures; a field of view that may be a few millimeters across would be enlarged to several inches on the video screen. Accordingly, the instrument must be moved very precisely in very small increments in order to approximate and treat the tissue being visualized. In fact, many times, the doctor must position the distal tip of the endoscopic instrument within a fraction of a millimeter of the desired location in order to achieve desired results. However, because of friction and backlash in the way the instrument passes through the endoscope, achieving this level of accuracy is difficult. For example, an endoscope several feet long may be positioned in the colon of a patient with the distal end of the endoscope tightly reflexed to visualize a particular area of the ascending colon. In such a position, the endoscope is bent into a very sinuous shape in multiple planes. Since the outside diameter of the endoscopic instrument is significantly smaller (e.g., 2.2 mm) than the inside diameter of the working channel (e.g., 3.2 mm), a large clearance space exists between the instrument and the channel. Likewise, there is a discrepancy between the outside diameter of the control member in comparison with the inside diameter of the endoscopic instrument tubular shaft. The outside diameter of a control member may be as small as 1 mm, while the inside diameter of the endoscopic instrument outer tube may be approximately 2 mm.
When the instrument is pulled back, the tension on the instrument causes the instrument to be pulled taut, and the instrument naturally assumes the shortest path through the working channel. When the instrument is pushed forward, friction causes it to assume the longest path through the channel (that is, the shaft of the instrument must “fill” the working channel before the distal end of the instrument begins to move). As a result, quite a bit of backlash (lost motion) is experienced by the doctor when the doctor tries to manipulate the distal end of the instrument. If it is necessary to pull the tip back a bit, the backlash must first be pulled out before the distal end can be retracted. If the doctor pulls the instrument back a little too far, the doctor must then push it several millimeters forward before there is any motion at all at the distal end. During this manipulation, the endoscopic instrument alternately assumes the longest-path and shortest-path positions within the working channel of the endoscope.
The situation with regard to the control member is analogous. As the control member is moved distally and proximately vis-à-vis the outer tube of the endoscopic instrument, the control member is respectively forced to fill the instrument tube, or be pulled taut, before the desired movement of the end effector assembly takes place. In both the movement of the endoscopic instrument through the working channel and the movement of the control member within the endoscopic instrument tube to operate the end effector assembly, it is desirable to minimize lag, or backlash. If this backlash can be reduced or eliminated, the manipulation of the distal end of the endoscopic instrument as a whole, or the operation of the device at the distal end of the endoscopic instrument operated by the control member, can be made much easier and more responsive, and the doctor can achieve his desired positioning or device operation more easily, rapidly, and precisely. In particular, a reduction in the backlash experienced in operating the end effector assembly with the control member would increase the precision of surgical techniques possible with the endoscopic instrument. However, this is not a simple problem to overcome for several reasons.
The backlash situations described above could possibly be reduced or substantially eliminated if the clearance between the outside of the control member and the inside of the tubular shaft of the endoscopic instrument were reduced. However, this is not a practical solution, because it is often necessary to inject fluid (or to operate suction) through the annular space between these two structures. If the control member were to substantially fill up the space within the tubular casing, the backlash would be reduced, but there would be greatly reduced ability to conduct fluid through the working channel around the instrument. In fact, because of the nature of fluid flow, as the aspect ratio of the annular clearance space (the ratio of the thickness of the fluid channel to its circumferential length) becomes small, the impedance to fluid flow grows disproportionately to the reduction in cross-sectional area of the fluid passage.
In addition, as the diameter of the control member approaches the inside diameter of the tubular casing, the area of contact between the instrument and the working channel becomes larger. This increase in contact area between these parts results in an increase in frictional drag on the control member when the doctor attempts to move it relative to the tubular shaft.